A compressed air supply system known in the art supplies dry compressed air to a load of a vehicle such as an air brake system or an air suspension. The compressed air supply system includes a compressor and a compressed air drying device. The compressed air drying device, which is arranged between the compressor and the vehicle load, dries compressed air. Nowadays, electronic control units (ECUs) are used to control compressed air drying devices (refer to, for example, patent document 1).
A compressed air drying device is provided with an air dryer, which includes a desiccant, and is referred to as an air dryer module. The compressed air drying device performs a loading operation and an unloading operation. In the loading operation, compressed air passes through the desiccant so that dry compressed air flows to the vehicle load. In the unloading operation, the flow of compressed air is reversed so that the compressed air flows to the air dryer to regenerate the desiccant of the air dryer. The air dryer module includes a discharge valve (exhaust valve) to discharge drainage. During regeneration, drainage collects in the discharge valve.
Under a cold condition in which the ambient temperature is below the freezing temperature, the drainage and water collected on the discharge valve may freeze. When forcibly opening the frozen discharge valve, excessive force may be applied to the discharge valve and cause an abnormality. Thus, the compressed air drying device of the prior art is provided with a heater to raise the temperature of the discharge valve.